1. Field of the Invention
This invention relates to conductive polymer compositions and electrical devices comprising such compositions.
2. Introduction to the Invention
Conductive polymer compositions and electrical devices comprising them are well-known. Such compositions comprise a polymeric component, and dispersed therein, a particulate conductive filler such as carbon black or metal. Conductive polymer compositions are described in U.S. Pat. Nos. 4,237,441 (van Konynenburg et al), 4,388,607 (Toy et al), 4,534,889 (van Konynenburg et al), 4,545,926 (Fouts et al), 4,560,498 (Horsma et al), 4,591,700 (Sopory), 4,724,417 (Au et al), 4,774,024 (Deep et al), 4,935,156 (van Konynenburg et al), 5,049,850 (Evans et al), 5,250,228 (Baigrie et al), and 5,378,407 (Chandler et al), and in pending U.S. application Ser. Nos. 08/085,859 (Chu et al, filed Jun. 29, 1993 , now U.S. Pat. No. 5,451,919), and 08/255,497 (Chu et al, filed Jun. 8, 1994), now U.S. Pat. No. 5,582,770. The disclosure of each of these patents and applications is incorporated herein by reference.
Such compositions often exhibit positive temperature coefficient (PTC) behavior, i.e. they increase in resistivity in response to an increase in temperature, generally over a relatively small temperature range. The temperature at which this increase occurs is the switching temperature T.sub.s and may be defined as the temperature at the intersection point of extensions of the substantially straight portions of a plot of the log of the resistance of a PTC element against temperature that lie on either side of the portion of the curve showing a sharp change in slope. The increase from the resistivity at 20.degree. C. (.rho..sub.20) to a peak resistivity (.rho..sub.peak, i.e. the maximum resistivity that the composition exhibits above T.sub.s or the resistivity that the composition exhibits at a specified temperature above T.sub.s) is the PTC anomaly height.
PTC conductive polymer compositions are particularly suitable for use in electrical devices such as circuit protection devices, heaters, and sensors that respond to changes in ambient temperature, current, and/or voltage conditions. For circuit protection device applications it is desirable that the composition have as low a resistivity and as high a PTC anomaly height as possible. A low resistivity allows preparation of small devices that have low resistance. Such devices need little space on a printed circuit board or other substrate and contribute little resistance to an electrical circuit during normal operation. In addition, because irradiation, heat treatment, and other processing steps that are often part of the preparation of the device increase resistance, a low resistivity material is desirable. A high PTC anomaly height allows the device to withstand the necessary applied voltage. The resistivity of a conductive polymer composition can be decreased by adding more conductive filler, but this generally reduces the PTC anomaly. A possible explanation for the reduction of the PTC anomaly is that the addition of more conductive filler (a) decreases the amount of crystalline polymer which contributes to the PTC anomaly, or (b) physically reinforces the polymeric component and thus decreases the expansion at the melting temperature.